106 Mr. Loyd A. Jones on a Method and Instrument 



at any instant and is therefore in the practical problem of an 

 object illuminated by natural light a complete specification 

 of the weather conditions at, a given time. This term will 

 be referred to as the ''Weather Coefficient," W, its evalua- 

 tion in terms of the other quantities being expressed by the 

 equation 



The variables to be considered are therefore R 2 and W y 

 and the necessary evaluations of visibility are of the form, 

 V&=/(R 2 ), W = a constant ; and V&=/(W), R 2 = a constant. 

 The tirst equation when properly formulated will make pos- 

 sible the computation of the variation in visibility due to a 

 variation in the value of the reflexion factor, for any specified 

 value of the weather coefficient; the second equation will give 

 the variation in visibility with the value of the weather co- 

 efficient for any object of definite reflexion power. 



Before proceeding with the formulation of these visibility 

 functions it is desirable, for the sake of clearness, to sum- 

 marize briefly the terms thus far defined which must be used 

 in the subsequent development of the theory. 



B! = Brightness of background. 



B 2 = Brightness of object. 



E2 = Illumination of object. 



R 2 = Reflexion factor of object. 



B v — Brightness of veiling glare which when superposed 

 over object and background will reduce the con- 

 trast to a just imperceptible value. 



V 6 = Brightness visibility. 



W = Weather Coefficient. 

 k — Constant contrast of the eye. 



Now, B„ must satisfy the equation, 



B,-f-B„ nx 



]vPB; =r ' () 



where c is a constant depending upon whether Bj is greater 

 or less than B 2 . 



I£ g>*, c =*; .... - • (2) 



or if! B x 1 ,o\ 



£<K c — -p (o) 



